Cellular data is typically communicated between devices via a network and one or more base stations. In general, the base station includes functionality for ensuring that the radio resource is used as efficiently as possible. For example, a radio base station may schedule device transmissions based on some suitable metric.
In some instances, however, devices may be configured for the transmission and receipt of direct communications with one another. For example, if a base station is not working properly, the devices may operate to communicate directly without the assistance of the radio base station. Such direct communication between devices may be referred to as device to device communication (or D2D communication). In 3GPP network, specifically, D2D communication may be called Proximity Services (ProSe). ProSe includes services that can be provided by the 3GPP system based on wireless devices being proximate to each other. Architectures and reference models are described in TS 23.303.
The resources used for supporting D2D communications may overlap with the resources used for supporting cellular traffic. Specifically, the D2D layer may use cellular uplink (UL) and/or downlink (DL) resources for the transmission of D2D data. For example, the resources used by the D2D layer may include physical resource blocks (PRB) that are used for the control of the D2D traffic by the cellular network node, such as an LTE eNB, the PRBs used by the D2D traffic, and the PRBs used for neighbor (peer) discovery by a D2D-capable user equipment (UE).
Investigations have shown that to meet the coverage requirements, each D2D transport block should be transmitted four times. The transmission procedure for D2D may typically follow the procedure for legacy transmissions. For example, a wireless device may first send a scheduling request (SR) to the eNB or other network node to indicate a need for resources. The wireless device may also send a buffer status report (BSR) with information on, among other things, the amount of data of different logical channels. In response, the network node can grant resources to the wireless device. The granting of the resources may be based upon considerations such as system load, traffic priority order, wireless device connection quality indications, the information included in the BSR and SR, and any other suitable information.
For D2D specifically, the wireless device may transmit a D2D-BSR on the PUSCH. The purpose of the D2D-BSR is to inform the network node about the amount of data the wireless device has on logical channels related to D2D. Although this makes it possible to reuse the existing BSR, it requires at least one logical channel group for D2D communication. If the wireless device is also configured with legacy LTE bearers and D2D discovery, the four existing logical channel groups may become a restriction.
The network node may set up periodic BSRs related to the validity time of the D2D grant for increased efficiency. The D2D grant may be transmitted on the PDCCH similar to legacy PUSCH grants. The purpose of the grant is to allow the wireless device to transmit data on the ProSe or other physical channel. The grant also allows the network node to control which wireless device gets to transmit at any given time and on what resources. Thus, the grant reduces communication interference and the possibility for collisions.
Before a wireless device can transmit the Scheduling Assignment (SA), the wireless device must receive a valid grant. For example the SA cycle can be 160 ms. In each cycle there may be up to four occasions for transmission of the SA. So, in short, every 40 ms there may be an opportunity to send an SA. As such, a wireless device may send an SA in one 40 ms opportunity of a given cycle and listen for other SAs during the other 120 ms of the same cycle. This means that a wireless device can send and receive D2D transmissions continuously if the data transmission patterns are orthogonal.
The sending of a SR may be triggered when the wireless device buffer includes data queued either for the cellular link or the D2D link. The existing scheme of scheduling requests follows a fixed transmission cycle assigned by the network and carries only 1 bit of information. Hence, when the wireless device sends a 1 bit SR, the network node cannot identify whether the request applies to a cellular link or a D2D link. As a result, the network node may not be able to schedule the wireless device in the appropriate priority order and assign the suitable resources accordingly.
Similarly, the network node may not be able to identify the BSR that follows the scheduling request as applying to the D2D link or the cellular link. As such, the network node may not understand whether the received buffer status information is used to convey the status for a cellular transmission or a D2D transmission.